Activation of the sympathetic nervous system produces many of the cardiovascular adjustments to exercise that match blood flow and oxygen delivery to the metabolic demands of the exercising muscles. In exercising muscle, the normal ability of sympathetic nerves to cause vasoconstriction is attenuated, a phenomenon termed functional sympatholysis. Dr. Thomas' previous work demonstrated that a key mediator of sympatholysis is nitric oxide (NO) derived from the neuronal isoform of NO synthase (nNOS) which is abundantly expressed in skeletal muscle cells. The expression of skeletal muscle nNOS is upregulated by the female sex hormone estrogen. Therefore, the central hypothesis of this application is that estrogen in females is a key mediator of functional sympatholysis because of its ability to induce nNOS expression. Dr. Thomas hypothesizes that in estrogen-deficient states the exercising muscles are ischemic due to decreased production of skeletal-muscle derived NO leading to unopposed alpha-adrenergic vasoconstriction. First, she will test the hypothesis that estrogen facilitates functional sympatholysis. In anesthetized female rats, she will measure hindlimb blood flow and muscle oxygenation with near infrared spectroscopy to determine if functional sympatholysis is attenuated in states of estrogen-deficiency such as sexual immaturity, or surgical ovariectomy. To determine if functional sympatholysis is similarly attenuated in women who are estrogen-deficient due to surgical or natural menopause, Dr. Thomas will measure forearm muscle oxygenation with NIR spectroscopy while recording muscle SNA with microelectrodes. She also will determine if in estrogen-deficient rats and humans, functional sympatholysis is restored by estrogen replacement, but not by progesterone replacement. Second, she will test the hypothesis that increased skeletal muscle expression of nNOS is a major mechanism mediating the beneficial effect of estrogen on functional sympatholysis. In rats, she will determine the effect of pharmacologic inhibition of NOS on functional sympatholysis in estrogen-replete versus estrogen-deficient animals. Dr. Thomas also will determine if skeletal muscle nNOS expression is reduced by ovariectomy or by an estrogen receptor antagonist, and is increased by estrogen replacement in ovariectomized rats. Finally, in female nNOS knockout mice she will determine if functional sympatholysis is attenuated and refractory to restoration by estrogen treatment. These rodent and human studies may elucidate a novel cardiovascular benefit of estrogen to enhance skeletal muscle perfusion and oxygenation in exercising muscles by opposing alpha-adrenergic vasoconstriction, thus providing further rationale for estrogen replacement therapy in women.